Bugs 6099 & 6472: Don't leave zombies and report errors

Update spawnve() to subprocess.check_output()
Pop an ErrorDialog if the system's open command fails in
open_file_with_default_application()

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@22417 4ae1f11a-8b86-4847-b8af-ab372f36d1fd